The production of circuit boards for certain types of data processing equipment in the computer industry requires the use of a combination of wave solder and solder paste, known as surface mount technology (SMT), to place components on the circuit boards.
As in any production technique, any cost savings are continuously being sought. To this end, any reduction in the number of chemicals and chemical steps will result in an improvement in manufacturing process and a subsequent savings of cost. One of the major cost elements in the production of certain circuit boards is the surface mount process itself, and a key component to the process is solder paste, which paste plays multiple roles in the process of surface mount technology.
The first step in surface mount technology is registration of the circuit board possessing the appropriate circuitry connection lines and copper pads for the attachment of the electrical components (such as capacitors and resistors, small outline integrated circuits and transistors, flat packs, and plastic leaded chip carriers). Solder paste is screened onto the circuit board using an appropriately patterned stencil, in all of the locations where connection of the components to the copper pads is required. After screening the solder paste onto the circuit board, the board is moved to a pick and place machine that places all the components on the prepared sites. The solder paste must be tacky enough to hold the components in their required location until the board is subject to reflow, at which time the solder paste flows to become the soldered connection for the components. Tack life is the term used for the length of time that a solder paste is sticky enough to hold components when they are placed in the screened sites. For a solvent-based solder paste the tack life can be a critical factor as the solvent begins to evaporate and dry the paste out.
Reflow is the term used to describe heating the solder above its melting temperature to give a soldered joint; for 63/37 tin/lead solder, the usual reflow temperature used is 185.degree.-230.degree. C. The process is repeated for the back side of the board as needed. Using rosin based commercial solder pastes, the board must be cleaned using a chlorofluorocarbon (CFC) solvent since the rosin flux used in commercial pastes is soluble in organic solvents only.
Thereafter, other components that require wave solder mounting are placed on the circuit boards. Wave solder consists of using an adhesive to hold components on the board and cleaning the copper areas on the circuit board with a liquid flux before passage over a "wave" of liquid solder. The final cleanings use water washes with and without added surfactants. Any necessary repairs are performed manually by operators after visual inspection of the completed circuit board, and is termed "rework."
The process of surface mount technology creates stringent material demands on the solder paste, which as noted above serves a variety of functions. Not only will the paste be making the inter-connections, it must be tacky and capable of holding the components in place before the reflow process. The solder paste is a materials system comprised of a suspension of solder powder in a flux. The flux component of the solder paste is the material that controls many of the requirements of the solder paste. The flux first must remove metal oxides from the solder powder to allow reflow of all the solder properly. It must contribute the right rheology for the screening process, and the paste must act as an adhesive to hold the components in place until reflow. Further, any residues left must be easily removed preferably by use of non-toxic materials, such as water, and any minor amount of residues must be non-conducting and non-corrosive.
Current solder pastes in use have a flux system composed of a complex mixture derived from a natural product based pine-tar distillate fraction (called rosin). This flux system leads to irreproducibility in results and is characterized by lot-to-lot and vendor-to-vendor variations. A variety of thickeners, activators and vehicles are added to the rosin to create the correct qualities of reflow and rheology. The rosin-based system contains volatile solvents that lead the solder paste to exhibit a short tack life (a few hours) as well as some screening problems if the material is left on the stencil too long, such as clogging the stencil by drying in the apertures. Additionally, a drying step is required before reflow to remove the solvents; otherwise problems (such as sputtering) during the reflow may be encountered due to the solvent volatilization. Also, many current solder pastes and fluxes have objectionable odors.
The rosin-based flux system requires an organic solvent to clean the flux residue after reflow. The best solvent for cleaning is a halogenated hydrocarbon. With the current restrictions on the use of halogenated hydrocarbons, this is a major problem with the commercial rosin based solder pastes.
The ideal solvent for flux clean-up is water due to its ready availability, low environmental impact, and low cost.
The use of Gafac RE-610, a trademark of GAF Corporation for a nonylphenyl polyoxyethylene ether phosphate, (whose chemical formula is shown in Table I), was reported in Anon, "Fluorocarbon Soluble Strong Acid Soldering Fluxes," Research Disclosure, Vol. 200, pg. 523-4, December 1980, as a flux component in wave soldering, as a wetting agent, but not in a solder paste formulation. Becker, G., Biverstedt, A. Tolvgard, A. "The Surfactant Flux--A New Flux for the Ozone Age," Hybrid Circuit, No. 16, pp. 66-68, May 1988, suggests the use of RE-610 in combination with amine or ammonium salts in an organic solvent to produce a liquid flux for wave solder that is fluorocarbon soluble. However, there is no suggestion of using a wave flux formulation as a solder paste flux since wave fluxes are more aggressive in general than solder paste fluxes. In addition, this was a solvent-based system and one of the objectives of this invention is to develop a solvent-free solder paste, and this reference does not suggest water as a solvent for cleaning; it emphasizes fluorocarbon solubility. Very significantly, this prior art does not discuss the critical aspects of solder paste, the good tack and tack-life, nor the needed screening properties.
U.S. Pat. No. 3,865,641 discloses a soldering composition to be mixed with an inorganic soldering flux such as phosphoric acid for use in soldering stainless steels. GAFAC RE-410 is suggested as a wetting agent. Amines are used as an inhibiting agent to inhibit the action of the acid flux. This is a solder paste formulation using a flux system of orthophosphoric acid, a small quantity of copper powder or copper salt, an amine as an inhibitor, and a wetting agent. Gafac wetting agents are specifically mentioned--but solely as materials for promoting uniform distribution of the inhibiting agent--not as a flux itself. Both the wetting agent and inhibitor are present in very small quantities (less than 4 percent each). The majority of the material is orthophosphoric acid (a very strong fluxing agent) and water. The amines used as inhibitors are amines possessing bulky substituents.
U.S. Pat. No. 4,151,015 discloses a water-soluble flux for plumbing soldering, which contains a water-soluble anionic or non-ionic surfactant and a corrosive water-soluble amine hydrohalide. This patent does not cover solder paste formulations.
U.S. Pat. No. 4,153,482 discloses a process for joining two metal surfaces together in which the surfaces are treated with a liquid flux. The liquid flux consists of an organic acid. The flux does not include any amine fluxing agents.
U.S. Pat. No. 4,168,996 discloses a non-water-soluble soldering flux combining rosins and many other components. A flux component is designed to release strong mineral acids or oxidizing agents at the use temperature and thus presents a corrosion hazard in electronics applications.
U.S. Pat. No. 4,298,407 discloses a solder powder coated with an organic flux rather than a solder paste. The residues are not water soluble.
U.S. Pat. No. 4,541,876 discloses a non-aqueous multi-component vehicle for metal paste therefore residues are not water-soluble.
U.S. Pat. No. 4,577,767 discloses a solder paste and vehicle. The paste is not water-soluble and requires an organic solvent wash.
U.S. Pat. No. 4,619,715 discloses a solder metal paste and vehicle, which contains a non-aqueous rosin binder. It is not water-soluble.
U.S. Pat. No. 4,728,023 discloses a process for vapor phase soldering utilizing a solder cream comprising solder metal and fluorinated tertiary alkylamines. The system is not water-soluble.
U.S. Pat. No. 4,180,419 reveals a solder flux formulation that is water-soluble using a polyhydric alcohol ester of phosphoric acid. This material is used as an application flux prior to hand-soldering or wave soldering, or as a solder wire core flux. No amine is disclosed. Though they claim use would be possible as solder paste, this has not been reduced to practice.
U.S. Pat. No. 4,196,024 discloses a soldering flux consisting of essentially mono- and/or di-alkyl phosphates or alkyl amine salts of these phosphates for use in a wave soldering system. The patent requires a foaming agent and organic carrier for the flux. There is no reference to a paste formulation and the flux is organic solvent soluble, not water-soluble.